Oxygen is the most abundant element in Earth, but its distribution and state (or fugacity) in the mantle and atmosphere have varied over time with important consequences for Earth's geologic and biologic history. For example, degassing of the mantle through melting and volcanism is thought to have affected the composition of the atmosphere.

To examine the mantle aspect of this coupled evolutionary process, Canil studied the partitioning of vanadium in mantle minerals and melts and in a suite of mantle-derived igneous rocks stretching back to the Archean. Vanadium can exist in several oxygenation states and partitions differently in melts in response to the local oxygen fugacity and mineralogy. The vanadium data imply that Earth's mantle was just as oxidized in the Archean (about 3.5 billion years ago) as it is today. Previous studies have supported a later increase of atmospheric oxygen abundance; thus, this result challenges proposed mechanisms for producing a gradual and later oxidation of Earth's atmosphere that are based on an early reduced mantle. — BH